The present invention relates to articles having self-cleaning layers and more particularly to an improvement of self-cleaning layers particularly for use in linings of various cooking utensils.
Of various methods for applying self-cleaning linings to ovens, the most typical method is disclosed in U.S. Pat. No. 3,266,477 to Stiles. Briefly stated, Stiles' method consists of sticking a suitable catalyst to porous substrates or walls of an oven. As a result, a particulate catalyst is exposed on the surfaces of a porous substrate so that the coated surfaces are easily susceptible to abrasion and wear by friction during use and consequently it is extremely difficult to maintain the catalytic oxidizing ability over a long period of time.
The method for solving this problem is disclosed in U.S. Pat. No. 3,547,098 to Lee. Briefly stated, the method consists of fusing to the operating surface of a cooking oven liner a porcelain enamel coating composed of a frit having had smelted homogeneously therethroughout a suitable oxidation inducing oxide so that the oxide may permanently be carried by the frit. As described in detail in the specification, raw materials of a frit and a catalyst are smelted at 2,350.degree. F., quenched (fritted) in cold water, and dried in a drier at 200.degree. F. The frit is milled using a clayless mill addition, and the enamel is milled, sprayed over a substrate and fused at 1,400.degree. F.
This method is very advantageous in that a catalyst may be homogeneously smelted and supported in a highly reliable manner, but some serious difficulties arise when the oxidation inducing metal oxides are heated in excess of a vitrification temperature (higher than 1,200.degree. C.) because almost all of the metal oxides react with a frit and are converted into spinel type compounds which are very stable chemically so that their catalytic activities are inevitably reduced to one half to one third of their inherent activities. In addition, a catalyst is homogeneously smelted in a frit so that it is not preferable to directly fuse the frit to a metal substrate because of the corrosion problem. As a result, the metal substrate must be coated with porcelain prior to the fusion step.
Furthermore, since a frit contains a relatively large amount of catalyst or oxidation inducing metal oxide the frit must be fused at a high temperature, higher than 700.degree. C., in order to obtain better bonding between the porcelain coated substrate and the frit. With a fusing temperature higher than 700.degree. C., it becomes extremely difficult to maintain the dimensional accuracies of parts of an oven within predetermined limits. Even when a working temperature is precisely controlled, a considerable number of oven parts are rejected because of their dimensional defects.
A further method for lining the desired surfaces of an oven with self-cleaning coatings is disclosed in U.S. Pat. No. 3,587,556 to Moreland. Briefly stated, this method consists of a mixture of glass frit and a catalyst both of which are available at the market and fusing this mixture to a panel or substrate provided with a base coat of porcelain enamel. A porous matrix layer of some depth is built up on the overlie in bonded relation with a base enamel layer, the porous matrix layer being formed by the glass frit particles bonding to each other and to the base enamel layer with the catalyst dispersed throughout the matrix.
Moreland's method is by far superior to Lee's method in that an oxidizing catalyst may be supported in an enamel layer in a more activated state and is more advantageous in that the catalytic activity may be preserved for a long period of time.
However, Moreland discloses his invention only with commercially available glass frit materials and catalysts and does not teach the composition of the glass frit materials especially. In other words, the composition of the glass frit materials with which his invention may be most effectively practiced is not disclosed at all.
The present invention was made to solve the above and other defects encountered in the prior art cooking utensils having self-cleaning walls or linings. After the extensive studies and experiments the inventors succeeded to provide frits which may be fused at a lower temperature and which may substantially overcome the pollution and food-poisoning problems.
Enamel frits which are used for lining self-cleaning cooking utensils must meet the following conditions:
(1) The slip containing an oxidizing catalyst must be fused at a temperature as low as possible. PA1 (2) The frit must not contain a compound or compounds which will cause pollution problems. PA1 (3) Strong bonding must be obtained between the substrate and the self-cleaning coating. PA1 (4) The self-cleaning coating must not cause corrosion of the substrate. PA1 (5) The self-cleaning coating must have a greater surface area and must exhibit excellent resistance to abrasion and wear. PA1 (6) The solubility to water of the self-cleaning coating must be as low as practicable.
The above conditions will be described in more detail. First it is preferable to fuse a frit containing an oxidizing catalyst at as low a temperature as practicable to a wall surface of a cooking utensil such as an oven because a high fusion temperature results in deformation of metal substrates with resultant unacceptable dimensional accuracies. In order to solve this problem, a metal substrate having a greater thickness must be used. Furthermore, with the increase in fusing temperature the capital cost of a furnace will increase and the fuel cost per part will also increase.
As to the oxidizing catalysts, almost all of the self-cleaning cooking utensils commercially available use copper oxides CuOx and manganese oxides MnOx in order to avoid food poisoning and because the walls of cooking utensils such as ovens are colored dark in order to make food soils invisible. Of various manganese oxides, it is electrolytically produced manganese dioxide .GAMMA.-MnO.sub.2 that has the catalytic property. .gamma.-Manganese dioxide is transformed as follows: EQU .gamma.-MnO.sub.2 350.degree. C. .beta.-MnO.sub.2 650.degree. C. .alpha.-Mn.sub.2 O.sub.3 950.degree. C. Mn.sub.3 O.sub.4
of these manganese oxides it is .gamma.-MnO.sub.2 that is most active and the largest surface area. In FIG. 5 there is shown the relationship between the ambient temperature (.degree. C.) and the surface area of .gamma.-MnO.sub.2. In the measurement, .gamma.-MnO.sub.2 was heated for 0.5 hour at each of the measured temperatures and the surface area was measured by the BET method. From FIG. 1 it is readily understood that it is preferable to reduce a fusing temperature as low as possible. In addition, strong bonding must be obtained between a self-cleaning coating and a metal substrate. In view of the above, commercially available frits to be fused to steel plates are not preferable to be used in lining self-cleaning cooking utensils because their fusing temperatures range between 830.degree. and 890.degree. C. Commercially available frits which may be fused at lower temperatures are frits containing lead, phosphoric acid or borosilicates, but the lead base frits are not preferable because of the food poisoning problem. The phosphoric acid base frits are also not preferable because the pollution problems arise in the production and the production itself becomes dangerous. In addition, their costs are expensive. From the standpoint of the pollution and food poisoning problems, the use of borosilicates base frits must be avoided because currently available frits contain pollutants such as PbO, Sb.sub.2 O.sub.3, CdO and the like.
As described above, currently commercially available frits are all not preferable because of the pollution and food poisoning problems.